Second Staircase Deadline: Getting on the Right Side of 30 September

13 September 2026 · 4 min read · Regulatory Update

On 30 September 2026, the second staircase provisions in Approved Document B Volume 1 take effect in England. The change itself is not news — it was published in 2024 — but the deadline now sits weeks away, and for every developer with a flatted scheme at or above 18 metres the practical questions are immediate: does the scheme fall under the new guidance, does it qualify for the transitional provisions, and if a second core is required, what does it do to the appraisal?

What Actually Changes

Blocks of flats with a top storey at 18 metres or more above ground level will need more than one common staircase. The height is measured to the upper floor surface of the top storey (Diagram D6), from ground level on the lowest side — not to the roofline or parapet, and rooftop plant areas are excluded. Two traps catch teams out. First, the threshold is inclusive: 18 metres or more, not more than 18 metres. Second, interlocked and scissor stair configurations count as a single escape stair, so a scheme that appears to have two stairs may still need redesign.

Below 18 metres, nothing is automatic — other escape provisions can still require more than one stair. And a second staircase can change the scheme that received planning permission, which deserves an early check even where a fresh application is not needed.

The Transition Window: Two Tests, Not One

Schemes can preserve the previous guidance if two things line up. First, before 30 September 2026 a building notice or initial notice must have been given, or a full plans building control application made. Planning permission is not enough — this is a building control test. Higher-risk building work adds a further constraint: the Building Safety Regulator's Gateway 2 approval is required before work starts, so the building-notice route is not a shortcut for in-scope HRB schemes.

Second, the work must have started and been sufficiently progressed — either before 30 September 2026, or within the 18-month period beginning on that date. For new buildings, "sufficiently progressed" means concrete poured for permanent trench, pad or raft foundations, or permanent piling started. Clearing the site, erecting fencing or excavating alone does not qualify. A timely submission without the programme to match does not bank the old guidance indefinitely, and dated evidence of the qualifying works should be agreed with the building control body and kept for funders.

Where the Money Goes

For schemes that must comply, the numbers are now well evidenced. Modelling published by Quantem on a representative 400,000 ft² residential building put the second staircase at approximately £1 million in additional construction cost — around £2.47 per ft² (roughly £26.50/m²) — before the knock-ons: additional lift provision or redistribution, pressurisation and smoke ventilation to a second core, extended corridors and additional facade area.

The quieter hit is efficiency. A second core erodes net internal area on every floor it serves, and international research on dual-stair mandates suggests saleable area losses that run to several percentage points. On a scheme where the land was bought on a single-core efficiency assumption, that lands directly on residual value — which is why funders are increasingly asking for the Part B position to be evidenced at due diligence, not discovered at Gateway 2.

What This Means in Practice

1. Measure, don't estimate. Check the Diagram D6 top-storey height on live schemes — including rooftop plant exclusions — before assuming the 18m threshold isn't triggered. Boundary cases deserve a measured survey, not a scale rule.
2. Audit your transition evidence. If relying on the old guidance, confirm the submission date, the foundations test and the programme. The 18-month window ends in March 2028 — schemes that cannot realistically start foundations inside it should redesign now, not hope.
3. Re-run the appraisal with the second core in it. Cost the stair, lifts, smoke control and facade, then test the NIA loss against GDV. On marginal schemes this is the difference between viable and not.
4. Watch the Gateway 2 interaction. For HRB schemes, design changes to incorporate a second stair after Gateway 2 submission mean revised approvals. Sequence the redesign before submission wherever the programme allows.
5. Document the golden thread. Keep dated records of building control submissions, foundation pours and BSR correspondence. Transitional status is an evidence question, and it will be tested at completion and on refinancing.

The second staircase rule rewards teams who treat 30 September as a design and programme milestone rather than a legal date. The schemes that struggle will not be the ones that redesigned early — they will be the ones that assumed a submission was enough.

Sources: MHCLG, Approved Document B Volume 1 (2026 amendments) and Impact Assessment on the Introduction of Second Staircases (gov.uk); The Building Regulations 2010 (as amended), transitional provisions; Quantem, "The Second Staircase Mandate: What It Means for Residential Development in England" (May 2026); Centre for Cities, "Dual staircase requirements". This article is for general guidance only and does not constitute legal or professional advice.